The invention pertains to machine vision and, more particularly, to methods for bandwidth reduction of multichannel images. The invention has application, for example, in facilitating machine vision analysis of color images.
Machine vision refers to the automated determination of characteristics of objects and other features shown in an image. It is often employed in automated manufacturing lines, where images of components are analyzed to determine placement and alignment prior to assembly. Machine vision is also used for quality assurance. For example, in the semiconductor electronics assembly industry, images of chips are analyzed to insure that leads, solder paste and other components do not overrun designated boundaries.
Machine vision has traditionally been performed on gray-scale images, rather than color images. Gray-scale images acquired of components on an electrical equipment assembly are typically processed using gray-scale machine vision "tools," i.e., software routines, that determine component orientation and location. Gray-scale processing has been justified on a number of grounds, including the relatively low cost of gray-scale cameras and minimal computational demands of gray-scale vision tools.
Color video cameras are gradually replacing gray-scale cameras as conventional image acquisition devices. Though potentially increasing the flexibility of machine vision systems, the processing of color images can be problematic. Color video cameras generate digitally-encoded images in which each pixel is rendered in not one, but multiple, spectral bands. In a conventional RGB (red, green, blue) color video signal, for example, each image is represented by three monochromatic (or gray-scale) images: one based on the red portion of the spectrum, one based on the green portion of the spectrum, and based on the blue portion of the spectrum. A scene represented by a "color" image is typically reconstructed by aligning and adding together the corresponding red, green and blue image components.
A color image is an example of a "multichannel" image. More generally, a multichannel image is an image that is made up of two or more images (which may be referred to as component images, or sub-images) each representing a respective (albeit, possibly overlapping) spectral component of a scene. For example, as described above, an RGB color image is made up red, green and blue component (or sub-) images. The pixels in each of those component images represent the intensities of the respective spectral band (i.e., pixels in the red component image represent the intensities of the "reds" that make up the scene, pixels in the green component image represent the intensities of the "greens" that make up the scene, and pixels in the blue component image represent the intensities of the "blues" that make up the scene). Of course, multichannel images--as used in this application--are not merely limited to the traditional red, green and blue spectral bands but may include additional or other spectral bands in the visible, infrared, ultraviolet, x-ray or other portions of the electromagnetic spectrum.
Many machine vision systems can not process multichannel images because, as discussed above, their vision tools are designed for processing, at a given time, only a single gray-scale image--not sets of gray-scale images (i.e., multichannel images). One prior art technique for overcoming this requires using the vision tools to process separately each channel of the multichannel image (e.g., color image). The results are combined to produce a composite answer. One drawback of this technique is that it greatly increases, i.e., by three-fold, the computation required to process an image. Another drawback is that it can adversely affect the contrast among features shown in the image.
An object of this invention is provide improved methods for machine vision and, particularly, for reducing the bandwidth of a multichannel image. As used herein, "bandwidth" refers to the number of channels, range of values, and/or range of frequencies necessary to represent an image, e.g., for transmission, processing or storage. For example, the bandwidth of a digitally-encoded image is the number of bits necessary to encode the intensities and/or colors of the underlying scene, i.e., eight bits in the case of a conventional gray-scale image and twenty-four bits in the case of a conventional RGB color image. Thus, as used herein, reducing the bandwidth of a color image refers, for example, to reducing the number of bits necessary to transmit, store and/or process a digital signal representing the underlying scene.
Another object of the invention is to provide such methods as can be used for permitting multichannel images to be processed with gray-scale machine vision tools.
Still another object of the invention is to provide such methods as can be used to reduce the bandwidth of multichannel images without adversely affecting useful information contained therein.
Yet still another object of the invention is to provide such methods as can be implemented on conventional digital data processors or other conventional machine vision analysis equipment.
Yet a further object of the invention is to provide such methods as can be implemented computationally, optically or otherwise.